Fix UTM display: remove required framebuffer tag, add serial output, ls app

Display fixes for UTM/Mac black screen issue:
- Remove MULTIBOOT_HEADER_TAG_FRAMEBUFFER from multiboot2 header (was required,
  requesting text mode depth=0 which confused GRUB on some platforms)
- Add COM1 serial port (0x3F8) output alongside debugcon for UTM serial capture
- Change VGA background from black to dark blue for diagnostics
- Add early canary write to 0xB8000 ('COS' in magenta) before subsystem init
- print_hex now outputs to both debugcon and COM1

New ls command and SYS_READDIR syscall:
- SYS_READDIR (10): reads directory entries via VFS
- VFS root listing: vfs_readdir handles '/' by iterating mount table
- apps/ls: lists CWD contents, appends '/' for directories
- apps/libc/syscalls.h: readdir() wrapper

README.md

@@ -56,7 +56,7 @@ Once a task is completed, it should be checked off.
 - [x] Create a `hello-world` app. It should print `Hello, World` to its own stdout. The kernel should route this to Qemu and to the VGA dispaly. Ensure this work.
 - [x] Implement the fork system call.
 - [x] Implement environment variables. Apps should be able to modify this, and it should be copied to new forks of an app.
-- [ ] Create a basic shell program `sh`. This shell must be able to start the hello-world app. It must include `cd` as a built-in to change the current working directory.
+- [x] Create a basic shell program `sh`. This shell must be able to start the hello-world app. It must include `cd` as a built-in to change the current working directory.
 - [ ] Create an `ls` app. It should list the contents of the current working directory, via the environment variable.
 - [ ] Create a devicefs vfs driver. The devicefs subsystem should allow drivers to create new block devices devices.
 - [ ] Create an IDE driver. It should enumerate all IDE devices, and expose them to the vfs driver as `hddN` or `cdN`, where N is a number that starts at 1 and increases for each new device. The `N` value should be calucated by the devicefs subsystem, not the IDE driver.